This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

This advisory was posted on December 7, 2007 7:05 AM

Today's Advisory:
The biggest hazard out there today is the potential to hit rocks, logs, or other partially buried objects that could result in a season ending injury. It may be possible to trigger a few new snow sluffs or small shallow soft slabs, especially in wind drifted areas near and above treeline. As we collect more data and snowpack observations we will begin issuing danger ratings as conditions warrant.

The first winter storm has arrived in the Sierra! Over the last 24 hours 14 to 18 inches of new 8% to 10% density snow fell above 7000' in the forecast area. Most of this snow accumulated late last night and this morning when snow fall rates reached 2 inches an hour. This storm should continue to move through the area today and start tapering off by midday allowing for some clearing this afternoon. Moderate southwest winds associated with this storm should have been enough to transport snow. Another smaller low pressure system should move down along the east side of the Sierra tomorrow. This low should keep the area cold by establishing a northerly flow. It may even bring a small amount of accumulation to the mountains. Unfortunately this northerly flow should also cause the winds to shift to the east and increase.

The new snow is falling on bare ground across most of the forecast area. Even though the total snow amount from this storm is not enough to bring an instant midwinter base and avalanche conditions to the Sierra, the wind loading resulting from the moderate southwest winds will begin to fill in many of the starting zones. The open slopes near and above treeline on N-NE-E aspects and gully features should experience the most wind loading and should be the first areas to fill in. Wind loading can increase snow deposition rates by 2 to 10 times the rate that snow is falling from the sky. As the snow accumulates on these wind loaded slopes, triggerable wind slabs may start to form. These slabs should start out as small, disconnected pillow drifts. However, the longer the winds continue to tranport snow and the more snow accumulates the larger and more extensive these wind slabs will grow despite a shallow snowpack in wind protected areas. Even a small slide can be disastrous at this time of year because of all the exposed hazards like rocks and trees. Remember to stay observant as you travel, look for indications of the current snowpack stability such as recent avalanche activity, wind loading, collapse, audible whumpfing sounds, shooting cracks, and/or test slope failure. Any of these signs are indications of current snowpack instability.

This new snow should be just enough to barley cover many rocks, down trees, and stumps. They will be lurking just below the nice white snow surface waiting to smash a knee, ski, board, or sled. Travel cautiously and slowly as it is a very long winter if you get hurt during this time of the year. Now is the time to put fresh batteries in your avalanche transceiver and practice your rescue skills with friends. Read an avalanche book and refresh your knowledge.

The biggest hazard out there today is the potential to hit rocks, logs, or other partially buried objects that could result in a season ending injury. It may be possible to trigger a few new snow sluffs or small shallow soft slabs, especially in wind drifted areas near and above treeline. As we collect more data and snowpack observations we will begin issuing danger ratings as conditions warrant.

Andy Anderson, Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet): 23 deg. F
Max. temperature at Sierra Crest past 24 hours: 29 deg. F
Average wind direction at Sierra Crest past 24 hours: Westsouthwest
Average wind speed at Sierra Crest past 24 hours: 20 mph
Maximum wind gust at Sierra Crest past 24 hours: 47 mph
New snow fall at 8,200 feet past 24 hours: 16 inches
Total snow depth at 8,200 feet: 18 inches
Mountain Weather Forecast For Today:
Continued snow this morning turning to snow showers this afternoon amounting to an additional 4 to 8 inches of accumulation.
Temperature forecast for 8,000 to 9,000 feet: 20 to 25 deg. F.
Ridgetop winds forecast for the Sierra Crest: West to northwest at 10 to 20 mph with gusts to 35 mph
Snowfall expected in the next 24 hours: 4 to 8 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today expect snow to continue through the morning then decrease and change to snow showers by this afternoon with daytime highs between 24 and 30 deg. F. An additional 3 to 7 inches of snow accumulation is forecast for today. Winds should be out of the west to northwest at 5 to 15 mph.

Tonight the snow should taper off into isolated showers under mostly cloudy skies. Overnight lows should be between 16 an 20 deg. F. The winds are forecast to shift more to the northwest and then to the northeast at 5 to 15 mph with gusts to 25 mph.

For Saturday, more isolated snow showers with up to 2 inches of accumulation. Daytime highs 20 to 26 deg. F. The winds should shift to the east and increase to 10 to 20 mph with gusts to 30 mph.

Above 8000 Feet:
Today expect snow to continue through the morning then decrease and change to snow showers by this afternoon with daytime highs between 20 and 25 deg. F. An additional 4 to 8 inches of snow accumulation is forecast for today. Winds should be out of the west to northwest at 10 to 20 mph with gusts to 35 mph.

Tonight the snow should taper off into isolated showers under mostly cloudy skies. Overnight lows should be between 14 an 17 deg. F. The winds are forecast to shift more to the northwest and then to the northeast at 10 to 20 mph with gusts to 30 mph.

For Saturday, more isolated snow showers with up to 2 inches of accumulation. Daytime highs 17 to 22 deg. F. The winds should shift from the northeast to the east and increase to 15 to 25 mph with gusts to 40 mph the afternoon.

The bottom line:

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature:	deg. F.
Max. temperature in the last 24 hours:	deg. F.
Average wind direction during the last 24 hours:
Average wind speed during the last 24 hours:	mph
Maximum wind gust in the last 24 hours:	mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.